Throughout much of the operational time of a vehicle, in particular a passenger automobile, the engine runs without driving the vehicle. For example, during intermittent shorttime stopping of the vehicle, e.g., due to traffic conditions, the engine, with the transmission in neutral, will continue to turn at the idling speed. Also, during deceleration and coasting, not only is the engine not driving the vehicle but it will itself be driven at a higher speed by the vehicle's forward momentum acting through the wheels of the car and transmission. Such coasting or deceleration occurs not only on driving downhill but whenever during travel the driver releases the gas pedal.
In both operating conditions, deceleration as well as idling, fuel delivered to the engine is wasted inasmuch as it is not utilized to drive the vehicle. In some cases of coasting and deceleration, braking by the engine is desirable for reasons of safety to boost the main foot brake, since the engine is used to dissipate kinetic energy. In cases other than travel down a steep incline, however, the fact that the engine continues to run when not powering the vehicle constitutes an extremely unprofitable utilization of fuel, a resource becoming scarcer and scarcer.